modification on notebooks from topic 9-13

[megaSpoon]

Since these tutorials are relatively similar in structure, I add some illustrative words in the Deep Forward NN tutorial to tell the user the basic structure they share. The gist of the modification is to make the readers easier to skip those repeated steps and focus on the differences once they had already learned one of these notebooks.

notebook change details:

1. make each headline in the notebook correct with respect to the chosen neural network
2. add subtitles to separate key steps from each other
3. add useful diagrams and links

[yangw1234]

Hi @megaSpoon, topic 10 seems to be left unchanged. Did you forgot to submit it?

[megaSpoon]

@yangw1234 previous CNN notebook fixed

[yangw1234]

LGTM